It is an object of the present invention to provide a pump device for pumping an active fiber of an optical amplifier. In particular, it is an object of the present invention to provide a pump device for coupling a pump radiation to an an optical amplifier adapted to be used in an optical transmission system, for example a wavelength division multiplexing (WDM) optical transmission system. The invention also relates to an optical amplifier that uses the above mentioned pump device.
Conventional optical fiber amplifiers include active fibers having a core doped with a rare earth element. Pump power at a characteristic wavelength for the rare earth element, when injected into the active fiber, excites the ions of the rare earth element, leading to gain in the core for an information signal propagating along the fiber.
Rare earth elements used for doping typically include Erbium (Er), Neodymium (Nd), Ytterbium (Yb), Samarium (Sm), and Praseodymium (Pr). The particular rare earth element or elements used is determined in accordance with the wavelength of the input signal light and the wavelength of the pump light. For example, Er ions would be used for input signal light having a wavelength of 1.55 xcexcm and for pump power having a wavelength of 1.48 xcexcm or 0.98 xcexcm; co-doping with Er and Yb ions, further, allows different and broader pump wavelength bands to be used.
Traditional pump sources include single mode laser diodes and multi-mode broad area lasers coupled to the active fiber over single mode and multi-mode pumping fibers, respectively, to provide the pump power. Single mode lasers provide low pump power, typically in the order of 100 mW. Broad area lasers, on the other hand, provide high pump power, in the order of 500 mW. These lasers of high output power, however, cannot efficiently inject light into the small core of a single mode fiber. Consequently, the use of high power broad area lasers requires the use of wide core and multi-mode fibers for pumping optical amplifiers. This non-active pumping fiber in turn typically inputs the pump power through a coupler and into the active fiber, for example into the inner cladding of a double-clad active fiber, acting as a multi-mode core for the pump power.
In a double-clad amplifier fiber, pump power is guided into the inner multi-mode cladding of the fiber from which it is transferred into a single mode core doped with an active dopant. The double-cladding fiber pumping mechanism is described for example in WO 95/10868. This document discloses a fiber optic amplifier comprising a fiber with two concentric cores. Pump power provided by multi-mode sources couples transversely to the outer core (equivalent to an inner cladding) of the fiber through multi-mode fibers and multi-mode optical couplers. The pump power propagates through the outer core and interacts with the inner core to pump active material contained in the inner core. This pumping technique is also described in U.S. Pat. No. 5,291,501, which illustrates a mono-mode optical fiber with doped core and doped inner cladding.
A well-known basic amplifying system includes a multi-mode pump source coupled to an amplification fiber, for example an ErNb doped doubleclad fiber, via a conventional fused fiber wavelength division multiplexer (WDM) type coupler. WDM couplers behave as multi-mode couplers for the pump power and transmit the single mode signals along the amplification fiber substantially without coupling to the pump fiber. During the pumping operation, most of the outer modes of the pump power are transmitted to the amplification fiber, leaving the inner modes of the pump power unused. In the case of a multi-mode or a double cladding amplifier fiber, a fused fiber coupler has a theoretical coupling coefficient directly proportional to the ratio of the areas of the two fibers constituting the coupler itself. In an ideal case for two identical fibers, the coupling coefficient is approximately 50%, but in practice it is in the range of 45-48%. This means that only about 45-48% of the total pump power passes from the pumping fiber into the inner cladding of the double-clad active fiber, while the remaining 52-55% remains in the pumping fiber.
Some systems use two optical fibers having different diameter of cores to improve the coupling coefficient of the multi-mode coupler. However, such arrangements often lead to a waste of power due to the difficulty in matching the tapering of two cores of different size.
To increase the coupling efficiency, the Applicant has considered the possibility of using micro optic couplers. Micro optic couplers couple optical beams using a wavelength selective mirror and a focusing lens system. With this construction, micro optic couplers obtain much better coupling efficiencies than traditional WDM couplers, typically in the range of 89%. Applicant has remarked that micro optic couplers have several drawbacks that limit their use for pump coupling in fiber amplifiers. In particular, if a single micro optic coupler is used upstream of the active fiber so as to feed the pump radiation to the active fiber in a co-propagating direction, the transmission signals passing through the coupler undergo a power loss that is much higher than the loss introduced by a fused fiber coupler and that may be excessive (particularly in consideration of the fact that the signals undergo the attenuation before being amplified and this leads to an increase of the noise figure for the amplifier). Alternatively, if a single micro optic coupler is positioned down-line with respect to the active fiber so as to feed the pump radiation to the active fiber in a counter-propagating direction, the signal to noise ratio undergoes a reduction which again may be excessive. Moreover, due to high coupling efficiency achievable by using a micro optic coupler, and then to the high pump power fed into the fiber, an inhomogeneous distribution of the population inversion is produced along the fiber.
More recent systems have attempted to recover the lost pump power in a conventional fused fiber coupler by means of different pumping schemes using fused fiber couplers. In particular, different solutions have been proposed that include a second optical coupler in addition to a first optical coupler positioned according to the above-described single-coupler arrangement. The second coupler is positioned at the opposite end of the active fiber with respect to the first coupler and is coupled to the first coupler through a multi-mode pump fiber so as to receive the residual pump power (i.e. the fraction of the pump power that has not been directly fed to the active fiber by means of the first coupler). The second coupler is then adapted to couple the residual pump power to the same active fiber in a counter-propagating direction, or to a different active fiber in a copropagating direction. The proposed pumping schemes using the above-mentioned technique to recover the pump power include only couplers of the fused fiber type. The Applicant observed that the addition of a second fused fiber coupler does not significantly improve the total pump power transfer over the single-coupler system described above. In fact, the second coupler receives from the pump fiber prevalently internal modes left over by the first coupling operated by the first coupler, and the transfer of the internal modes into the active fiber is inefficient.
EP patent application No 97114622.0 in the name of the Applicant proposes a technique to improve the total coupling efficiency in the above two couplers pumping schemes. The improvement is obtained by interposing, along the pump fiber connection coupling the first and the second coupler, a mode scrambler, i.e. a device that operates a scrambling of the inner modes on the residual pump radiation so as to regenerate a high number of external modes that can be efficiently transferred into the active fiber through the second coupler. Under ideal circumstances, 50% of the pump power signal enters the active fiber at each of the couplers. This would lead to a total coupling efficiency for the coupling system of 75%. In practice, however, the total coupling efficiency is close to 68%.
EP patent application No. 97114620.4 in the name of the Applicant proposes a different solution for the same problem, consisting in the use of two unequal couplers manufactured by a fusion biconical tapering technique. In practice, the first and the second couplers have different fusing and tapering amounts with the active fiber so as to achieve a better coupling efficiency. This further pumping solution allows a total coupling efficiency of approximately 66%.
It is an object of the present invention is to provide a pump device that is adapted to couple pump radiation to the active fiber (or active fibers) with a better coupling efficiency with respect to the known optical amplifiers.
The Applicant has found that a very high coupling efficiency can be achieved by providing a first coupler with a very low insertion loss for the signals (even if it has a very low pump coupling efficiency) and a coupler having a high pump coupling efficiency (even if it has a relatively high insertion loss).
In particular, the Applicant has found that a very high coupling efficiency can be obtained by means of a double-coupler arrangement in which a first coupler, adapted to receive a pump radiation from a pump source and to couple it to the active fiber, has an insertion loss for optical signals not greater than 0,2 dB, and a second coupler, adapted to receive a residual fraction of the pump radiation from the first coupler and to couple it to the active fiber, has a coupling efficiency for the pump radiation not less than 70%.
The couplers can feed the opposite ends of a single active fiber section (in which case the second coupler counter-pumps the fiber) or can feed two separate sections of active fiber. In this case the second coupler preferably pumps the second section of fiber in a co-propagating direction, while the first coupler can pump in either direction the first section.
The Applicant has in particular found that a double-coupler arrangement in which said first coupler is a fused fiber coupler and said second coupler is a micro optic coupler can provide a total coupling efficiency up to 85%, much higher than the maximum coupling efficiencies achievable with known arrangements.
According to a first aspect, the present invention relates to a pump device for coupling a pump radiation into an active fiber of an optical amplifier, said optical amplifier being adapted to amplify optical signals, said pump device including a multi-mode optical fiber to receive and convey a multi-mode pump radiation, a first optical coupler for optically coupling a first fraction of said pump radiation to said active fiber, a second optical coupler that is optically coupled to said first coupler to receive from said first coupler a second fraction of said pump radiation and that is further optically coupled to said active fiber to feed to said active fiber at least part of the second fraction of said pump radiation, characterized in that said first optical coupler has an insertion loss for said optical signals less than or equal to 0.2 dB and said second optical coupler has a coupling efficiency for said pump radiation of at least 70%.
Preferably, said first optical coupler is a fused fiber coupler and said second optical coupler is a micro optic coupler.
Preferably, the sum of the optical power of the first fraction of said pump radiation and of the optical power of said at least part of the second fraction of said pump radiation is more than 75% of the total optical power of said pump radiation. More preferably, said sum is at least 85% of the total optical power of said pump radiation.
Preferably, the pump device further comprises a pump optical fiber optically coupling said second optical coupler to said first optical coupler; said pump optical fiber being a multi-modal optical fiber adapted to transmit optical radiation without substantial energy transfer between modes.
Preferably, said first optical coupler has a first access fiber; a second access fiber that is a multi-modal fiber and is optically coupled to said multi-mode optical fiber to receive said pump radiation; a third access fiber that is of the same type of said first access fiber adapted to be coupled to said active fiber to feed to said active fiber the first fraction of said pump radiation; and a fourth access fiber, of the same type of said second access fiber, into which said second fraction of said pump radiation is conveyed; and said second coupler has a first access fiber that is a multi-modal fiber optically coupled to said fourth access fiber of said first optical coupler to receive the second fraction of said pump radiation; a second access fiber that is a double-cladding fiber adapted to be coupled to said second active fiber to feed to said second active fiber said at least part of the second fraction of said pump radiation, and a third access fiber for conveying said optical signals.
Preferably, said first access fiber of said first coupler is a single-mode fiber adapted to be coupled to an optical input to receive said optical signals and said third access fiber of said second coupler is a single-mode fiber adapted to be coupled to an optical output to feed to said optical output said optical signals.
According to a second aspect, the invention relates to an optical amplifier including an optical input for the input of optical signals, an optical output for the output of said optical signal, an active fiber interposed between said input and said output and adapted to amplify said optical signals, a pump source for generating a pump radiation and a pumping device according to the above to optically couple said pump radiation to said active fiber.
Preferably, said active fiber is a double-cladding fiber.
Preferably, said active fiber comprises two fiber sections, each coupled to a respective one of the two couplers.
According to another aspect, the invention relates to an optical amplifying unit including two optical amplifiers according to the above arranged in series.
Preferably, said optical amplifying unit further includes a pre-amplifier arranged in series with said optical amplifiers.
Preferably, said optical amplifying unit further includes at least one noise rejection filter arranged in series with said optical amplifiers.
According to another aspect, the invention relates to an optical transmission system including an optical transmitting unit adapted to transmit an optical signal, an optical receiving unit to receive said optical signal, an optical fiber link optically coupling said transmitting unit to said receiving unit, characterized in that it further includes an active fiber positioned along said optical fiber link to amplify said optical signal, a pump source to generate pump radiation and a pump device according to the above to couple said pump radiation to said active fiber.
According to a further aspect, the invention relates to a method for coupling a pump radiation into an active fiber adapted to amplify optical signals, comprising the following steps:
guiding an optical signal;
guiding a multimode pump radiation;
inputting said optical signal and said pump radiation to said active fiber; said optical signal being input with a predetermined insertion loss and said pump radiation being input so as to feed a first power fraction to said active fiber and to obtain a residual power fraction;
inputting said residual power fraction to said active fiber with a predetermined coupling efficiency so as to feed a second power fraction to said active fiber;
characterized in that said insertion loss is lower than or equal to 0.2 dB and said coupling efficiency is at least 70%. Preferably, the sum of said first and second power fractions is more than 75%, and more preferably at least 85%, of the optical power of said pump radiation.
The foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. The following description, as well as the practice of the invention, set forth and suggest additional advantages and purposes of this invention.